Why does cloth sewing not work correctly on this imported SVG?
The main concept to learn is that faces do not bend unless they are subdivided.
The first image shows that the object has no vertical subdivisions. How do you expect to bend it?
Rambling answer:
SVGs are converted to curves when imported to blender. When you convert curves (or text) to mesh the resulting geometry is usually triangulated or quite irregular, useless for the most part.
A workaround is not to fill the shape and fill it yourself. To do that, just import the SVG but before converting to mesh, set the fill on the curve to None, so that you only keep the outline.
Proceed to convert curve to mesh and you will be left with some vertices connected with edges outlining the object.
In Edit Mode select those edges and do &quot;Grid Fill&quot; and play with the settings for grid fill until you get an evenly divided mesh comprised of quad faces.
The resulting object should bend the way you need it to.
Here is an example borrowed from a different answer:

Note that for grid fill to work you need an Even number of vertices on the outline.
Also, in the shape you show on the question, you will need more subdivisions at the bottom, on the straight line.